Problem witjh Mojotone 5E8-A - Treble pot acting as Volume control

[nadles]

Hi all,

I originally posted in the maintenance, troubleshooting and repair forum, but now realise that i should have posted here.

I've just built a Mojotone 5E8-A kit and it sounds wonderful, however it has a couple of issues.

1. The treble pot acts as a volume control (affects both channels). With the treble pot on 0 the volume is very low and can barely be heard. start turning the treble pot and the treble increases as does the volume.

2. The amp is not as loud as expected. With the volume and treble on 12 (full) the amp is loud, but not as loud as i would expect from a 40W amp. The sweet spot for the amp is Vol 7, Treble 7 and bass 3. At these settings the 5E8-A is a little louder than my Deluxe Reverb clone with the vol on 3.5. I was expecting the volume to be closer to my JTM45 clone.

3. Filter caps discharge themselves as soon as the power is switched off. With the amp on, the filter caps have 455 V. Within approx 10 seconds of turning the power off the Filter caps read 8-11 VDC. (I've tested this both with the meter on the cap as it discharges, and without the meter - results are the same.).

I've tried isolating the problem and believe its in the preamp section. I inserted an Audio signal (tape deck) onto the wiper of the treble pot and controlled the volume using the tape deck controls. This output signal was loud.

The amp is wired using the original Fender schematic and layout. I've used the 8ohm tap on the OT and 240V (Australia) input on the PT.

Any suggestions on where to look next to isolate the problem would be appreciated.

Thanks in advance.

Here's voltages on the valve pins. Although they are higher than on the fender schematic most are within 20%. Those marked with * are outside +/- 20%. The heaters are 6.8v.

V1
1: 144.6v
2: .0004v
3: 2.41v
6: 141.1
7: 0
8: 2.41

V2
1: 141.2v
2: .0001v
3: 2.42v
6: 139.5
7: 0
8: 2.40

V3
1: 129.9v
2: .0002v
3: 2.068v
6: 266.8
7: 130.3v
8: 132.6

V4
1: 223.4v
2: .0003v
3: 1.78v
6: 278.1
7: 24.18v
8: 66.4 * (50v on schematic)

V5
3: 445.6v
4: 447.6v
5: -55.9v * (-42v on schematic)
6: -55.85 * (-42v on schematic)

V6
3: 444.6v
4: 445.7v
5: -55.8v * (-42v on schematic)
6: -55.9v * (-42v on schematic)


Rectifier Heater voltage across pins 2 and 8 is 5.5VAC
V7
2: 7
4: 358
6: 358
8: 5.7

V8
2: 6.9
4: 358
6: 358
8: 5.5

[Jazz P Bass]

Measure the resistance of the treble pot legs to ground.
All legs should not be anywhere near it.
If you do get a low reading, something is wired incorrect.

[nadles]

Thanks for the response.

With the Bass pot on 0, Treble pot legs to ground read:
1: 1.176M
2 (middle): 228.4K
3: 228.4K

With Bass pot on 12, Treble pot legs to ground read:
1: 1.482M
2 (middle): 536K
3: 536K

Turning the volume pots doesn't affect the resistance readings on the treble pot legs.

[Jazz P Bass]

Those readings seem correct.
Now, is there any Vdc on the tone circuit?(there should be none)
Try to measure that.

[nadles]

There is very low Vdc on the treble pot:

Treble on 0
Lug1 - 0.007V
Lug2 - 0.002V
Lug3 - 0.002V

Treble on 12 (full)
Lug1 - 0.002V
Lug2 - 0.002V
Lug3 - 0.002V

These were steady readings. I checked the Bass pot and the meter flickered between 0 and 0.001v

[loudthud]

The Fender "E Series" tone controls are very different from most other Fender amps. They offer such a wide range of control, many guitar players have trouble adjusting the amp between songs. It takes a lot of getting used to.

The ohm readings on the treble pot seem correct. Have you checked the 0.01uF cap that goes to ground? Try a smaller cap, 0.005uF or 0.002uF.

[nadles]

Thanks Loudthud. I removed the .01uF cap from the third leg of the treble pot and the volume increased significantly sounds like a 40W amp now.

I tried a .0047uF (using jumper leads) and it caused exactly the same issues as when the 0.01uF was in place: low volume and treble controls volume.

Could it be that the pot is bad?

[nadles]

Here's some pics of the wiring before removing the 0.01 uF cap from the Treble pot.

[loudthud]

The pot could be bad or just the wrong value. Try measuring end to end and each end to the middle termiinal with the shaft turned to half rotation.

[nadles]

Thanks thud,

End to end it reads: 0.953M

With the shaft in the middle position:
Lug 1 to Middle: 0.856M
Lug3 to middle : 99.7K

I tried a 0.0022uF cap on the treble pot. it didn't seem to affect the volume as much, but the amp sounded very muffled until the treble was on 5. Using the .01uF cap the amp sounds muffled with low volume until treble on 3, then treble and volume increase as the shaft is rotated.

[nickb]

It might be that nothing is coming through the bass path. Disconnect the middle of the treble pot as a test. You should still get plenty of signal with the bass at 0. You should lose highs as you increase the bass.

[nadles]

It might be that nothing is coming through the bass path. Disconnect the middle of the treble pot as a test. You should still get plenty of signal with the bass at 0. You should lose highs as you increase the bass.

Thanks Nick. I disconnected the 220K resistor from the middle lug of the treble pot and wow! Huge boost in volume - The amp is now JTM45 loud!!.

Reattaching the 0.01uF cap (or a 0.0022uF) between the treble pot and ground cuts volume as per original problem (with 220k resistor disconnected).

[nadles]

It might be that nothing is coming through the bass path. Disconnect the middle of the treble pot as a test. You should still get plenty of signal with the bass at 0. You should lose highs as you increase the bass.

update.

I reattached the the 220K resistor (with a jumper lead) to check the effect turning the bass pot has with the treble cap removed.

Bass pot 0: - low output as per original problem, but bright sound not muffled as it is with treble cap attached.
Bass pot 0-8 - volume increases, but not as loud as with 220k resistor disconnected.
Bass pot 9: volume about as loud as with 220k resistor disconnected
Bass pot 10 - 12: highs roll off no real change in volume.

[Steve A.]

It's kinda late to do anything about this now but did you confirm the values of all the resistors and caps with a meter? I usually do that but then again I do have OCD...

Steve Ahola

[nadles]

It's kinda late to do anything about this now but did you confirm the values of all the resistors and caps with a meter? I usually do that but then again I do have OCD...

Steve Ahola

Ha ha - yes I did, but I'll recheck the values of the resistors by colour codes. I'm colour blind, but my wife isn't